Re: VPN usin dynamic IPs
As Ede and Bob mentioned this is no problem at all.
Here's the quick FortiGuard DDNS method:
Under Network / DNS at the bottom you'll see a toggle for "FortiGuard DDNS". Toggle that on, select your WAN interface that your VPN is configured on, select "Use Public IP address", select one of the three dynamic domain names under Server (fortiddns.com, fortidyndns.com or float-zone.com) and then type in your unique name that will form the first part of the FQDN that is shown at the bottom.
You can then configure your clients to VPN to that FQDN.